Investigating ecosystems

• Quadrat


  A square frame made of wood or metal used to study the distribution of small organisms
• Quadrat


  • One of the simplest ways of sampling a habitat
  • A square frame enclosing a known area, e.g. 1 m2
• How to use a quadrat

  1. Generate random coordinates of the site

  2. Put the quadrat at the random location

  3. Count the plants and animals inside it, ignore anything outside the quadrat

  4. Repeat the process again in different parts of the field

  5. Calculate the average number of individuals per unit square for each species and measure the area of the habitat under study to estimate the density of each species
• To compare how common an organism is in two sample areas

  1. Place a 1 m2 quadrat on the ground at a random point within the first sample area

  2. Count all the organisms you're interested in within the quadrat

  3. Repeat steps 1 and 2 lots of times

  4. Work out the mean number of organisms per quadrat within the first sample area

  5. Repeat steps 1 to 4 in the second sample area

  6. Compare the two means
• The larger the sample size the better
• Mean


  total number of organisms/number of quadrats
• Comparing organism abundance


  • 2 daisies per m2 in the shade
  • 22 daisies per m2 (lots more) in an open field
• Estimate population sizes by scaling up from a small sample area:

  • To work out the population size of an organism in one sample area, you need to work out the mean number of organisms per m2 (if your quadrat has an area of 1 m2, this is the same as the mean number of organisms per quadrat, worked out above). Then just multiply the mean by the total area of the habitat:
• Use capture-mark-release-recapture to estimate population sizes:

  1. Capture a sample of the population and mark the animals in a harmless way.
  2. Release them back into the environment (and give the animals time to redistribute into the population).
  3. Recapture another sample of the population. Count how many of this sample are marked.
  4. Then estimate population size with this equation:

  population size = number in first sample x number in second sample / number in second sample marked
• Pitfall trap 

  • is a steep-sided container, with an open top, which is sunk into the ground.
  • It’s used to trap ground-dwelling insects.
• Keys are used to identify creatures

  Key 

  • is a series of questions that you can use to figure out what an unknown organism is.
  • useful when carrying out sampling as they help you to correctly identify organisms you find.
• Key


  A tool used to identify an unknown organism by answering a series of questions or statements that narrow down the options until the organism is identified
• Using a key


  1. Start at question 1

  2. Answer the question based on the mystery organism

  3. Use the answer to narrow down the options

  4. Answer more questions to further narrow down the options

  5. Eventually identify the one possible species
• Key structure


  • Can have statements rather than questions
  • Followed by a number of options
• • Part of a key is shown on the right
  • It can be used to identify types of organisms that might be found on the ground in woodland
• Transects 

  • are used to investigate distribution
  • this method is used to record exactly where each species or type of organism is found.
• Line transect

  • can be made from a string marked at regular intervals
  • it is stretched across the habitat you want to examine.
  • all the organisms touching the string are recorded with their distance from one end of the line called the 'origin'
• Belt transect

  • a better method than line transect.
  • is made by laying out two parallel strings, perhaps one metre or less apart and record the plants between them  or placing quadrats continuously or in regular intervals along a transect line the distribution and relative abundance of different species along the belt transect can be studied.
  • the results can be represented by a 'kite' diagram or histogram.
• The distribution of organisms is affected by abiotic factors such as temperature, moisture level, light intensity, and soil pH.
• How to measure abiotic factors

  1. Thermometer - measure the temperature in different places

  2. Soil moisture meter - measure the level of moisture in some soil

  3. Indicator liquid - water

  4. Electronic pH monitors - produce a more accurate pH value for the sample being tested

  5. Light meter - measure light intensity
• Indicator species


  Organisms that are sensitive to changes in their environment that can be studied to see how polluted an area is
• Indicator species


  • stonefly larvae
  • freshwater shrimps
• Polluted water


  Can lead to eutrophication
• Eutrophication


  Reduces the concentration of dissolved oxygen in the water
• The distribution of indicator species can be used to assess pollution